<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    .container {
      width: 0px;
      height: 0px;
      border: 20px solid red;
      border-top-color: transparent;
      border-left-color: transparent;
      border-right-color: transparent;
    }
  </style>
</head>

<body>
  <div class="container"></div>
</body>

</html>